Vertical semiconductor deep ultraviolet light emitting diodes on a nanowire-assisted aluminum nitride buffer layer

Vertical light-emitting diodes (LEDs) have many advantages such as uniform current injection, excellent scalability of the chip size, and simple packaging process. Hitherto, however, technologically important semiconductor aluminum gallium nitride (AlGaN) deep ultraviolet (UV) LEDs are mainly through lateral injection. Herein, we demonstrate a new and practical path for vertical AlGaN deep UV LEDs, which exploits a thin AlN buffer layer formed on a nanowire-based template on silicon (Si). Such a buffer layer enables in situ formation of vertical AlGaN deep UV LEDs on Si. Near Lambertian emission pattern is measured from the top surface. The decent reflectivity of Si in the deep UV range makes such a configuration a viable low-cost solution for vertical AlGaN deep UV LEDs. More importantly, the use of such a thin AlN buffer layer can allow an easy transfer of device structures to other carrier wafers for vertical AlGaN deep UV LEDs with ultimately high electrical and optical performance.


Results
The MBE growth and characterization of the AlN epilayer is described first. The schematic of the structure is shown in Fig. 1a, which starts with a thin (1-2 nm) AlN layer, followed by a thin (50-100 nm) GaN nanowire layer. Figure 1b shows the reflection high-energy electron diffraction (RHEED) pattern during the growth of the GaN nanowires. Regularly arranged dots are seen, suggesting a 3-dimensional (3D) growth. This RHEED feature is different from the arcs as often observed from self-organized GaN nanowires, which indicates the improvement of the nanowire vertical alignment with respect to the substrate, due to the use of the thin prenanowire AlN layer 47,48 .
The start of the growth of the AlN epilayer led to the RHEED pattern transition from being spotty to being streaky. The Al-rich growth condition was confirmed by the presence of excess Al through the following observations: (1) the Al shutter open and close test, wherein the close of the Al shutter led to an increase of the RHEED intensity and the opening of the Al shutter led to an intensity decrease; and (2) the observation of the RHEED  www.nature.com/scientificreports/ 2 × 6 reconstruction (Fig. 1c, d) during the growth, which suggests the presence of an Al adlayer 49,50 . Such a RHEED pattern also reflects that the AlN epilayer is Al-polar. In addition, the narrow, bright, and streaky RHEED pattern indicates a highly smooth surface. Figure 2a shows the optical image of the as-grown AlN epilayer, an optically smooth surface can be seen. The surface of the as-grown AlN epilayer was further examined by scanning electron microscopy (SEM). The images were taken at a tilting angle of 45°. Figure 2b shows a typical SEM image, highlighting a very smooth surface. The inset of Fig. 2b shows an SEM image that manifests the cross section, with the Si substrate, GaN nanowire layer, and AlN epilayer clearly seen. The surface was further examined by atomic force microscopy (AFM). A typical AFM image is shown in Fig. 2c. For such AlN epilayers, a root-mean-square (RMS) roughness of around 0.4 nm can be obtained, which is comparable to the typical metal-polar AlGaN thin films grown on sapphire and AlNon-sapphire template 51 . This RMS roughness is also significantly improved compared to our previous studies 41,42 .
To confirm the Al-polar polarity of the AlN epilayer, we have performed potassium hydroxide (KOH) etching experiments. In the experiments, 11.2 mol/L KOH solution was heated up to 70 °C, and the sample was entirely placed in the solution, followed by de-ionized (DI) water cleaning. The SEM image of the AlN epilayer after KOH etching is shown in Fig. 2d. It is seen that hexagonal pits, rather than hillocks, appear on the surface, confirming that the AlN epilayer is Al-polar [52][53][54] . This Al-polar AlN epilayer can further enable metal-polar AlGaN epilayers grown on top. The benefit is, as the opposite side of the metal-polar surface is N-polar, which can be selectively etched by KOH, it thus enables the removal of unwanted AlGaN epilayers (e.g., additional AlGaN buffer layers for material quality improvement), as well as the roughening of the surface through which the light comes out (for flip-chip devices). Indeed, the AlGaN epilayers to be described below are metal-polar as confirmed by KOH experiments.
We further demonstrate vertical AlGaN deep UV LEDs using such AlN epilayers as buffer layers on Si. The schematic of the device structure is shown in Fig. 3a, which consists of double heterojunctions (DHs) with an i-Al 0.4 Ga 0.6 N active region (~ 15 nm) and p-and n-Al 0.7 Ga 0.3 N cladding layers (~ 30 nm each). These Al contents were estimated from the room temperature photoluminescence (PL) experiments from AlGaN epilayers grown separately on such AlN buffer layers. Figure 3b shows the room temperature PL spectra measured from the AlGaN epilayers intended to be used as the active region and cladding layers for the devices. It is seen that PL emission at around 240 nm and 280 nm are measured. Assuming the PL peak energy approximately to be the bandgap energy and the bowing factor to be 1 55 , the Al contents in the active region and cladding layers were estimated to be ~ 40% and ~ 70%, respectively. It is also noted that only a single PL emission peak is seen for both the active region and cladding layers, suggesting a uniform alloy composition.
For device fabrication, we did not use chemical etching to isolate devices with different sizes. The isolation was obtained by the limitation of the current spreading length in the vertical injection scheme. Our calculation indicates that under an injection current density of 0.1 A/cm 2 , with the best reported p-AlGaN resistivity (Al content of 70%) and the largest ideality factor 56-58 , as well as the present p-AlGaN layer thickness, the maximum www.nature.com/scientificreports/ current spreading length is on the order of tens of μm. Therefore, by placing p-contact with a separation on the order of several hundred μm, devices can be naturally isolated.
The room-temperature I-V characteristics under a continuous-wave (CW) biasing are shown in Fig. 3c. At forward voltages of 9 V and 12 V, the forward currents were 6 mA and 23 mA, respectively, which are improved compared to the previously reported laterally injected AlGaN thin film UV LEDs on Si at a similar operating wavelength, e.g., Ref. 33 . Moreover, the device size dependent current further suggests a uniform current injection. Illustrated in the inset of Fig. 3c is a device size dependent current under a forward voltage of 12 V, and it is seen that the current increase is proportional to the device size increase, indicating a uniform current injection.
The electroluminescence (EL) spectra under different injection currents are shown in Fig. 4a. It is seen that the EL emission occurs at 298 nm. With the change of the injection current from 2 to 20 mA, no noticeable EL emission peak blueshift and full-width-at-half-maximum narrowing are observed. The EL emission wavelength is longer than what we expect, and the reason is being investigated. Figure 4b shows the EL spectra measured with a UV-VIS spectrometer, intended to study the defect EL emission. A parasitic EL emission peak at around 400 nm is seen, which could be originated from the deep levels in AlGaN alloys 59,60 . Its intensity, however, is significantly lower compared to the main EL emission peak. Figure 4c shows the light output power as a function of the injection current under a CW biasing. A typical LED behavior is seen, i.e., the light output power increases nearly linearly with the injection current. Under an injection current of 100 mA, a light output power of 0.3 μW was measured. An optical image of the light emission is shown in the inset of Fig. 4c, wherein a bright and uniform emission can be seen.
The emission pattern of such LEDs is also studied. In this regard, the optical fiber was tilted at various angles with respect to the axial direction (the growth direction) for the light detection, as illustrated in Fig. 4d. The emission pattern is shown in Fig. 4e by open circles. The ideal Lambertian emission pattern is also shown by the solid curve. It is seen that the device shows a near Lambertian pattern, suggesting the nature of surface emission.

Discussion
In this work, we have demonstrated vertical AlGaN deep UV LEDs on Si. Such devices are made on a special AlN buffer layer that is formed with the assistance of a nanowire-based template. As Si is a decent reflector in the deep UV range, such vertical devices offer a low-cost solution for vertical semiconductor deep UV LEDs and a potential benefit of in situ integration to other electronics on Si, and are suitable for low-power applications. Further improvement on the electrical performance for such devices is expected by optimizing the electrical doping; and further improvement on the light output power can be expected by optimizing the p-contact and adopting more complicated device designs such as using quantum wells and electron blocking layers. More attractively, as the thickness of the AlN buffer layer is very thin, it can be removed easily by chemical wet etching (same for the nanowires, as the sidewall of nanowires grown by MBE is N-polar, e.g., Ref. 61 ), which allows the transfer of device structures grown on top to other carrier wafers for the achievement of vertical AlGaN deep UV LEDs with ultimately high electrical and optical performance. Therefore, this work enables a practical path for high performance vertical semiconductor deep UV LEDs. Photoluminescence experiments. A 213 nm pulsed laser with a pulse width of 7 ns was used to excite the sample. The emitted light from the sample top surface was collected by an optical fiber, which was further coupled to a deep UV spectrometer.
Device fabrication. Metal bilayer Ni (7 nm)/Au (7 nm) was used for p-contact, which was fabricated by standard photolithography and metallization processes. The size of p-contact defines the size of the device. Colloidal Ag conductive adhesive was used on the backside of n-Si substrate as the n-contact.
Device characterization. The electroluminescence was collected by an optical fiber from the device top surface, and both deep UV and UV-VIS spectrometers were used for the spectral analysis. The light output power was measured by a Si photodetector, which was placed roughly about 5 mm above the device top surface. The device was unpackaged.

Data availability
The data is available upon reasonable request to the corresponding author.